1. Field of the Invention
The invention relates to rotary drag-type drill bits for use in drilling holes in subsurface formations and of the kind where cutting structures are mounted on a bit body having an axis of rotation. One common form of bit has a shank for connection to a drill string, a plurality of circumferentially spaced blades on the bit body extending outwardly away from the central axis of rotation of the bit, and a plurality of separate cutting elements mounted along each blade. A passage in the bit body supplies drilling fluid to nozzles in the surface of the bit for cleaning and cooling the cutters.
2. Description of Related Art
The invention is particularly, but not exclusively, applicable to drill bits in which some or all of the cutters are preform cutters formed, at least in part, from polycrystalline diamond or other superhard material. One common form of cutter comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the cutter, bonded to a substrate which is usually of cemented tungsten carbide.
The invention is also applicable to drill bits where the cutting structures comprise particles of natural or synthetic diamond, or other superhard material, embedded in a body of less hard material. The cutting structures may also comprise regions of a larger substantially continuous body comprising particles of superhard material embedded in a less hard material.
The bit body may be machined from solid metal, usually steel, or may be molded using a powder metallurgy process in which tungsten carbide powder is infiltrated with a metal alloy binder in a furnace so as to form a hard matrix.
The outer extremities of the cutters or other cutting structures on the drill bit define an overall cutting profile which defines the surface shape of the bottom of the borehole which the bit drills. Preferably the cutting profile is substantially continuous over the leading face of the bit so as to form a comparatively smooth bottom hole profile.
It is desirable, when designing a drill bit of the above kind, to be able to make a reasonably accurate prediction of the rate of wear of the cutting structures and, in particular, to compare the likely rates of wear of different cutting structure arrangements. The present invention provides an improved method for doing this.
It is common practice to use computers to model and analyze bit designs and methods of analysis have previously been proposed and used for predicting cutter wear. Such analysis is usually carried out by constructing a computerized model or representation of a particular bit design, a computer algorithm being designed to perform a series of steps on the computerized model of the bit in order to predict cutter wear. However, while existing methods may provide useful comparisons in wear rate between designs of bit where cutters are of the same type, size and shape, the existing methods cannot provide useful wear comparisons between bit designs having different cutter types, sizes or shapes. Existing methods are also usually dependent on the rate of penetration of the drill bit.
Also, existing methods generally assume that the wear rate of the cutting structures is substantially constant over the life of the bit, which may not be the case.
The present invention therefore sets out to provide a new method of determining the wear characteristics of a rotary drag-type bit which is independent of the type, size and shape of the cutting structures, and is also independent of rate of penetration (ROP). In a preferred method, other factors affecting wear rate may also taken be into account. Essentially, the method consists in evaluating for each design of drill bit a volume of cutting structure material, for example the volume of diamond or other superhard material, which is "available" to be worn away, irrespective of the shapes and dimensions of the cutting structures which provide such material, the wear rate being a function of such volume. The method is also applicable to determine the volume of cutter material which has actually been worn away in an actual used drill bit, so that the wear characteristics of an actual bit design can be compared with those of another actual bit, or with a proposed new design of bit.